ABSTRACT
The most consistent pull-off force studies involving CH3-terminated monolayers have been done in liquid environments. Several studies have reported pulloff forces ranging from 0.5 to 5 nN in ethanol solution [1, 3, 6, 12]. The variation in the measured forces was attributed to the size of the interacting probe. The SAM-ethanol interfacial free energy was calculated according to the Johnson-Kendall-Roberts (JKR) theory [17] to be in the range of 1.0-3.0 mJ/m 2
in these studies. The pull-off force values measured between CH3-terminated surfaces in water
were much less reproducible [1, 12, 16]. Complications in pull-off force measurements arise in an aqueous environment due to capillary effects associated with formation of gas bridges between hydrophobic surfaces. Submicroscopic air bubbles can nucleate on hydrophobic surfaces, which then form air bridges between the probe and the substrate [16].
